There are many factors that you should be aware of when adding new cats to your family. I strongly suggest both cats be fixed before you introduce them. If both of the animals are fixed, it is simply a matter of separation and scent switching. This means switching areas so each can smell the other without stress. Switching beds, toys, litter boxes and even crates will help with this. Then, you can allow them to spend time together while you look on. When they seem content with each other's areas and smells, they should be ok. Fights are natural, but temporarily separating them will help. Be sure the litter boxes are not covered and that there is at least one box for each cat so they can't corner each other.